The present invention relates to a gumming device. The invention can be used to good advantage to apply an adhesive substance to lengths of paper tape in machines for the tobacco industry to which the description below refers but without thereby restricting the scope of the invention.
In particular, the invention relates to a gumming roller device used in a cigarette filter tip attaching machine to apply a layer of adhesive substance to a continuous paper tape that is subsequently cut into individual lengths or strips to be used to attach the filter tips to cigarettes.
A gumming device used to apply a layer of adhesive to a continuous paper tape from which the connecting strips of filter paper for a cigarette filter tip attaching machine are cut is disclosed by United Kingdom patent No. 2,067,436, this gumming device comprising a pair of horizontal, counter-rotating rollers, placed side by side and elastically pressed against each other along a zone of reciprocal tangency. One of the two rollers is a dosing roller and the other a gumming roller whose surface, being essentially cylindrical, is tangent to the continuous tape.
The facing portions of the two cylindrical surfaces, just above the zone of tangency between the dosing roller and the gumming roller, define a space above which there is located a nozzle connected to the outfeed end of a pipe from a tank containing an adhesive substance.
The adhesive accumulates inside the said space, which extends along the entire contact generator between the two rollers, so as to constantly supply the gumming roller. While the gumming roller rotates in a defined direction, its outer cylindrical surface transfers a layer of adhesive to the continuous paper tape, the thickness of the layer being adjusted by the pressure between the two rollers.
Practice has shown that a gumming device of the type described cannot cope with the high operating speeds reached by the latest generation, of cigarette makers, causing the two rollers to turn at very high speeds, because it is unable to correctly transfer a layer of adhesive onto the surface of the of the continuous paper tape to be gummed. This is because the mass of adhesive substance inside the said space is subjected to a high-speed wave-like motion by the cylindrical surfaces of the two rollers, giving rise to strong turbulence with high pressure gradients within the mass of adhesive. This turbulence traps air inside the adhesive creating bubbles and causing the adhesive to splash when the air bubbles burst. Moreover, when the air bubbles come into contact with the cylindrical surface of the gumming roller, they prevent the adhesive from being properly distributed on the surface and creating breaks and uneven patches in the layer of adhesive transferred by the gumming roller.
As a result, parts of the surface of the continuous paper tape to be gummed have no adhesive on them at all.
The aim of the present invention is to provide a gumming device that overcomes the above mentioned disadvantages.
Accordingly, the present invention provides a gumming device comprising a first and a second roller, with horizontal, parallel axes, in contact with each other at a zone of tangency along a common generator, defining a space between the respective cylindrical surfaces just above the zone of tangency; and means for feeding an adhesive substance into the space, the device being characterized in that the adhesive feed means comprise means designed to distribute the adhesive located and shaped in such a way, in relation to the rollers, as to partially occupy the space.